Optical disk devices are used for the storage of computer-prepared data and have recognized value in their ability to store large quantities of data. The media for use in such devices is reactive to bursts of light, such as may be produced by the rapid switching of a semi-conductor laser. In order to write data on optical media, the laser power must be controlled at a fairly high power level, in order that the media can be altered in accordance with digits of data. In reading the data back, the laser power level is controlled to a lower level so that the media is not altered by the laser beam but the reflected light indicates the presence or absence of media alterations, that is, digits of data.
Optical media is of two general types, media which can be written only once and media which can be written, erased, and written again. Write-once media (WORM) is permanently altered when write power levels are produced by the laser beam. Erasable media, such as magneto-optic (MO) media, is not permanently altered when data is written. In the MO media, the magnetic orientation of the reactive material is altered in the writing process, and in the erasing process, the magnetic orientation is recorded.
When reading MO data from an optical disk, the remanent magnetization of one or the other polarity rotates the linear polarization of a reflected light beam creating P and S polarization components. By detecting these components of the light beam, an MO data signal is generated.
To readback data contained on write-once media, the reflected light beam intensity is modulated by the permanent condition of the disk. By detecting the intensity of the reflected light beam, a ROM signal is generated in accordance with the WORM data.
In operating an optical disk system, it is necessary to identify the particular sector and track upon which the laser beam is directed. That identification information is included in a sector header and stamped onto the disk itself. The user area, that is the data area, may be either write-once or erasable, but the sector header will always be permanent. Thus, even when writing or reading MO media, it is necessary to generate non-MO signals, ROM signals, from a sector header area.
In the writable region used for MO customer data, the readback signal is detected differentially for the best signal to noise ratio. Each component of the polarized light, P and S, is focused on a separate photodetector. The MO signal is the difference current or voltage signal generated by the polarized P and S light signals impinging on each detector. One technique for generating the difference signal is to amplify each photocurrent by a current or voltage amplifier and then take the difference for MO signal detection. For generating a ROM signal, the output of either amplifier or the sum signal of both amplifiers can be used.
For MO optical disks, as data density and the readback speed increase it becomes difficult to switch from reading the reflectivity (ROM) signal from the sector header to the detection of MO signals in the data area without encountering significant noise when reading the first few digits of the data area. Similarly, when switching from the data area to the next sector header, the switching time and the transients resulting from the switch create excessive noise when attempting to read the sector header. In order to accommodate the increased speed and data densities required for optical disks used in computer storage applications, the detection circuit of the instant invention has been developed to eliminate switching times and transients resulting from switching.